This invention pertains generally to guidance systems for missiles and particularly to systems of such character which utilize radar signals to derive command signals for guiding missiles during flight.
It is known in the art that a so-called "semiactive" radar guidance system may be used to derive the requisite guidance signals for a missile in flight toward a target. In a typical system of such a type, radar echo signals from a target being tracked by an illuminator (which may be either an airborne or a ground radar) and radar signals from the illuminator are received in the missile and demodulated. The demodulated signals then are processed to provide the requisite guidance signals during flight of the missile. Additionally, the radar echo signals may also be utilized to determine when ordnance carried by the missile should be armed and detonated to realize the highest probability of lethal damage to the target.
In order that the receiving arrangement in the missile may be operated satisfactorily in the presence of noise or interfering signals, it is common practice to provide a so-called "inverse" receiver for the amplification of the radar echo signals from a target. Briefly, an inverse receiver is a receiver having an intermediate frequency (I.F.) amplifier section with an extremely narrow pass band. Such section is fed by a beat frequency signal obtained by mixing the radar echo signals with the output of a local oscillator carried on the missile. Obviously, such an amplifier may be operated to good effect only when the frequency of the radar echo signals is known and the frequency of the output of local oscillator is adjustable to an offset frequency equal to the beat frequency to which the amplifier may respond.
It is relatively difficult to provide, using components which may be subject to instability in frequency and susceptible to frequency modulated (FM) noise, a satisfactory inverse receiver for use in a missile controlled by a semiactive radar guidance system. One accepted way for solving the technical problems involved is to provide, in addition to an inverse receiver for radar echo signals, a so-called "rear" receiver. Such a receiver includes an antenna oriented rearwardly of the missile and illuminated during flight by the airborne or ground radar (whence comes the designation "rear" receiver) and means for continuously controlling, by an automatic frequency control arrangement, the frequency of the local oscillator in the missile in accordance with the frequency of signals received from such radar. Additionally, the automatic frequency control arrangement is used to change the frequency of the output of the local oscillator, thereby compensating for any change in frequency of the radar echo signals due to any Doppler velocity between the missile and target to maintain the frequency of the beat frequency signal within the pass band of the I.F. amplifier.
While adequate performance of a semiactive radar guidance system using a rear receiver has been achieved in many instances, there are several reasons for improving the way in which the control of the frequency of the output of the local oscillator in a missile using such a guidance system is effected. First of all, if the complexity added by the rear receiver may be reduced, a concomitant decrease in cost may be achieved. Further, a reduced complexity of the rear receiver may increase the reliability of the system. Most importantly, perhaps, is the fact that if the frequency of the local oscillator in the missile may be controlled without requiring continuous illumination by the airborne or ground radar during flight, constraints on the intercept course of the missile may be relaxed significantly. That is to say, if continuous illumination is not necessary to control the frequency of the local oscillator in the missile, a semiactive guidance technique may be used with a single radar tracking a target, even though the intercept course of the missile is not within the beam (or any sidelobe) of such radar.
It is well known to use monopulse techniques in a receiver in a missile to derive angle errors in yaw and pitch with a high degree of precision. According to known monopulse techniques, radio frequency signals received as echo signals from a target are first processed to obtain a sum signal, a yaw error signal, y.sub.e, and a pitch error signal, p.sub.e. After downconversion of such processed signals and amplification in three different channels, the signals are multiplexed, normalized, demultiplexed and detected. While such an approach is satisfactory in operation, relatively complex circuitry is required. It would, obviously, then be advantageous to accomplish the same end as a conventional monopulse receiver with relatively simple circuitry.
An altogether different problem is encountered, especially when the illuminator is an airborne radar, with conventional semiactive radar guidance systems for missiles. If there are aircraft in formation, each one carrying a radar to control missiles, a high probability exists that mutual interference may be experienced. For example, if during the initial portion of the flight of a missile, the relative positions of the mother aircraft (meaning the aircraft from which the missile was launched) and the missile are such that radar signals from the mother aircraft enter through a sidelobe of the antenna pattern of the rear receiver and the relative positions of a sister aircraft (meaning another aircraft in the formation) and the missile are such that radar signals from the sister aircraft enter through the main lobe of the antenna pattern of the rear receiver, the local oscillator in the missile may be caused to "lock" on the radar signals from the sister aircraft. Obviously, then, the radar in the mother aircraft is ineffective to provide the radar echo signals required to derive guidance signals for the missile.